The proposed research is a continuation of our previous studies on the regulation of the excitability and motility of the smooth muscle of the female reproductive tract, and consists of 3 projects: I. The ionic mechanisms underlying the electrophysiological changes in the circular muscle of the rat uterus that occur during pregnancy and that are essential for partuition. II. The interactions between circular and longitudinal muscles in the rat uterus at different times in pregnancy and their possible significance. III. The relative sensitivities of the two muscle layers to agents believed to be involved in partuition, i.e. oxytocin and prostaglandins. Experiments will be done at the organ and cellular levels and will involve measurements of contractions and membrane potentials. The objective of the work is a more complete understanding of the factors involved in the maintenance of pregnancy and the initiation of labor.